For cell phones and select mobile devices constrained to operate on batteries, traditional fixed-supply power amplifiers (also referred to herein simply as traditional amplifiers) are being superseded by radio frequency (RF) amplifiers that utilize envelope-tracking (ET) technology for greater energy efficiency. This allows for longer periods of operation on a single charge. A traditional amplifier operates most efficiently in compression. However, since its power supply signal (e.g., supply voltage) is fixed, compression cannot be maintained as an input signal being amplified drops below a peak power value to which the amplifier must be tuned to prevent excessive clipping. As a ratio of the peak amplitude to the average amplitude (i.e., crest factor) grows for the input signal, the traditional amplifier becomes increasingly inefficient.
The implementation of envelope-tracking technology allows an amplifier to remain in compression as signal levels fluctuate. This is accomplished by providing the amplifier with a variable supply voltage that tracks the input signal being amplified. When the input signal drops, the supply voltage drops with it, and efficiency becomes a function of how closely the two signals are synchronized. Generating the power supply signal for the amplifier involves filtering the power supply signal. Filtering, however, affects the time alignment between the two signals due to frequency-dependant phase shifts that are introduced by filter components. It is in this area, i.e., filtering control and time synchronization, that significant improvements to the state of the art are possible.
Accordingly, there is a need for a method and apparatus for controlling the bandwidth of a filter circuit or compensating for changes in the bandwidth of the filter circuit within a system having two integrated circuits.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention. In addition, the description and drawings do not necessarily require the order illustrated. It will be further appreciated that certain actions and/or steps may be described or depicted in a particular order of occurrence while those skilled in the art will understand that such specificity with respect to sequence is not actually required.
The apparatus and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.